1. Field of the Invention
The present invention relates to a printed circuit board mounted on an electronic device, and more particularly to a printed circuit board in which undesired electromagnetic wave radiation (hereinafter referred to as electromagnetic wave radiation) is suppressed.
2. Description of the Related Art
In recent years, as the technology of high-higher speed and higher density electronic apparatus progresses, undesired electromagnetic wave radiation from the electronic apparatus causes problems such as malfunction of electronic devices disposed near the apparatus (for example, fluctuations in image on TV screen). Electronic device manufactures are developing technology to suppress such electromagnetic wave radiation.
The electromagnetic wave radiation results from high-frequency signals transmitted through a printed circuit board. Thus, in order to suppress the radiation of electromagnetic waves from an electronic device, it is effective to apply a countermeasure to a printed circuit board to be mounted on electronic device.
For this reason, various techniques have been developed to suppress electromagnetic wave radiation from printed circuit boards, and a need exists for a technique to suppress the electromagnetic wave radiation at a higher level.
Effective suppressing measures taken for a printed circuit board itself can eliminate, for example, the need to provide a shield plate for covering the printed circuit board, or the need to use an expensive shield cable for a cable connected to the printed circuit board, thereby allowing a significant reduction in cost of products.
As a technique for suppressing the electromagnetic wave radiation for a printed circuit board, Japanese Patent Laid-open No. 9-283974 proposes a low EMI multi-layered circuit board which suppresses strong electromagnetic wave radiation due to resonance of a power source layer and a ground layer in the board.
As shown in FIG. 1, the low EMI multi-layered circuit board comprises a board structure in which first dielectric layer 203 is sandwiched between power source layer 201 and first ground layer 202 to form capacitance C1, second dielectric layer 205 is sandwiched between power source layer 201 and second ground layer 204 to form capacitance C2, and resistor 206 is connected between first ground layer 202 and second ground layer 204.
As to power source layer 201 and first ground layer 202, a serial circuit including resistor 206 and a capacitor (capacitance C2) is connected between them. For power source noise at high frequency, they act as a structure in which only resistor 206 is connected between the two layers 201, 202.
Resistor 206 holds Q value (an index representing the ratio of stored energy to consumed energy) of resonance caused between power source layer 201 and first ground layer 202 at a low level, thereby making it possible to suppress strong electromagnetic wave radiation due to the resonance.
In addition, since the low EMI multi-layered circuit board is configured to suppress the electromagnetic wave radiation by means of the whole of the board, it has, for example, an advantage of easier mounting design around an LSI package.
The low EMI multi-layered circuit board, however, requires a high dielectric layer and a second ground layer in addition to a typical ground layer. Thus, it has problems that it is not obtained by easily changing board design based on normal rules and that the special board structure results in an increase in manufacturing cost.
As another technique for suppressing the electromagnetic wave radiation, Japanese Patent Publication No.7-46748 proposes a printed circuit board which suppresses strong electromagnetic wave radiation due to resonance of a power source layer and a ground layer in the board.
As shown in FIG. 3, the circuit board is configured such that a power source layer within printed circuit board 300 is divided into main power plane 301 and sub-power plane 304, and power source terminals 305 of LSI 303 are electrically connected to sub-power plane 304 and capacitors 306, and power source terminals 305 are electrically connected to ground systems 302 of main power plane 301 through ferrite bead 308 and capacitor 307 (for FB).
The configuration in which power source terminals 305 are electrically connected to main power plane 301 through a xcfx80 type decoupling circuit comprising capacitor 306, ferrite bead 308 and capacitor 307 (for FB) serves to prevent power source noise from leaking to main power plane 301 from LSI 303, suppressing the electromagnetic wave radiation from main power plane 301.
Printed circuit board 300 has an advantage of easy implementation in terms of design and manufacturing steps since it can be obtained by slightly changing a typical board structure.
However, when an LSI with a number of pins is mounted, a problem occurs that an increased size of sub power plane 304 may cause resonance between sub power plane 304 and the ground layer.
The printed circuit board also has a problem of higher manufacturing cost due to the use of the expensive ferrite bead.
As another technique for suppressing the electromagnetic wave radiation, the present inventors have proposed a printed circuit board which suppresses strong electromagnetic wave radiation caused by resonance of a power source layer and a ground layer in the printed circuit board.
As shown in FIG. 2, the printed circuit board is configured such that power source terminal 405a of IC 404 is electrically connected to common power source wiring 406 of the printed circuit board through a decoupling circuit comprising first capacitor 401, power source wiring 402 and second capacitor 403. The electrodes of first and second capacitors 401, 403 on the ground side are electrically connected to via holes 407a, 407b (ground conductors), respectively.
Ground terminal 405b of IC 404 is electrically connected to via hole 407c (ground conductor).
In the printed circuit board, the characteristic impedance of the power source wiring is set to be sufficiently higher than the impedance in the first capacitor, and the length thereof is set to approximately one quarter the wavelength at the upper limit frequency in a frequency range in the electromagnetic wave radiation. Thus, the power source wiring serves as an inductor with high impedance within the frequency range in the electromagnetic wave radiation to suppress high frequency noise which leaks from the power source terminal to the common power source wiring, resulting in suppression of electromagnetic wave radiation due to resonance of the common power source wiring.
In this manner, since the printed circuit board includes the decoupling circuit formed of the inexpensive capacitors and the power source wiring, the problem of increased cost is not remarkable, and this technique allows low manufacturing cost while harmful electromagnetic wave radiation can be suppressed.
The printed circuit board, however, applies the decoupling circuit to all of the power source terminals in mounting an LSI, so that routing of signal wiring may not be easy in wiring design, in which case a problem occurs that a reduction in design cost cannot be achieved.
To solve the aforementioned problems, it is an object of the present invention to provide a printed circuit board at lower cost and produce higher radiation suppressing effect by improving the printed circuit board shown in FIG. 27 to allow efficient arrangement of decoupling circuits for an LSI having a number of power source terminals.
To achieve the aforementioned object, a printed circuit board according to a first aspect of the present invention, including ground conductors and a power source conductor, on which an electronic component provided with two or more power source terminals is mounted, comprises: two or more first capacitors for electrically connecting the two or more power source terminals to the ground conductors, respectively, a first power source wiring for electrically connecting electrodes of adjacent first capacitors of the two or more first capacitors on the power source terminal sides, a second power source wiring for electrically connecting at least one of electrodes of the two or more first capacitors on the power source terminal side to the power source conductor, and a second capacitor for electrically connecting the power source conductor electrically connected to the second power source wiring to the ground conductor, wherein the first and second power source wirings have characteristic impedance three times or more higher than the impedance of the first and second capacitors in a frequency range in which undesired electromagnetic wave radiation occurs, and the first and second power source wirings have lengths which are equal to or larger than a value obtained by multiplying 20 mm by the wavelength reduction rate of the printed circuit board and are equal to or smaller than a value obtained by multiplying one quarter-wavelength at the upper limit frequency at which the undesired electromagnetic wave radiation occurs by the wavelength reduction rate.
Since such a configuration enables a reduced number of capacitors and reduced space for mounting, routing of signal wiring is facilitated in wiring design, thereby making it possible to provide a printed circuit board at reduced design cost and produce high effect of suppressing electromagnetic wave radiation.
According to a second aspect, the printed circuit board is configured such that the second power source wiring is electrically connected to the electrode of the first capacitors on the power source terminal side electrically connected to a power source terminal other than the power source terminal with the largest noise out of the power source terminals electrically connected with the first power source wiring.
In this manner, since the second power source wiring is electrically connected to the electrode on the power source terminal side of the first capacitor at least one capacitor away from the first capacitor electrically connected to the power source terminal with the largest noise, a multi-stage decoupling circuit is formed for a power source terminal with large power source noise. Thus, it is possible to provide a printed circuit board capable of further reducing the number of the capacitors and the mounting space with higher effect of suppressing electromagnetic wave radiation.
According to a third aspect, the printed circuit board is configured such that the first power source wiring electrically connects the electrodes of the first capacitors on the power source terminal sides electrically connected to a plurality of the power source terminals provided on one side of the electronic component.
The first power source wiring connects the first capacitors for each side of the electrical component (for example, an electronic component including an active element) to allow easier wiring design.
According to a fourth aspect, the printed circuit board is configured such that the first power source wiring is electrically connected to the electrodes on the power source terminal sides of the first capacitors adjacent to each other at each corner of the electronic component.
The first power source wiring also connects the first capacitors, for each corner of the electrical component in this case (for example, an electronic component including an active element) to allow easier wiring design similarly to claim 3.
According to a fifth aspect, the printed circuit board is configured such that, for the power source terminal susceptible to the power source noise, a decoupling circuit comprises the first capacitor electrically connected to the power source terminal, a third power source wiring electrically connected only to the first capacitor and to the power source conductor, and a third capacitor electrically connected to the power source conductor and to the ground conductor, wherein the third power source wiring has a characteristic impedance three times or more higher than the impedance in the third capacitor in a frequency range in which the undesired electromagnetic wave radiation occurs, and the third power source wiring has a length which is equal to or larger than a value obtained by multiplying 20 mm by the wavelength reduction rate of the printed circuit board and is equal to or smaller than a value obtained by multiplying one quarter the wavelength at the upper limit frequency at which the undesired electromagnetic wave radiation occurs by the wavelength reduction rate.
In this manner, individual decoupling circuits are applied to the power source terminal susceptible to noise and to the other power source terminals, thereby making it possible to provide a printed circuit board which can effectively suppress electromagnetic wave radiation in which the influence of power source noise is avoided.
According to a sixth aspect, the printed circuit board is configured such that, for the power source terminal involving large power source noise, a decoupling circuit comprises the first capacitor electrically connected to the power source terminal, a fourth power source wiring electrically connected only to the first capacitor and to the power source conductor, and a fourth capacitor electrically connected to the power source conductor and to the ground conductor, wherein the fourth power source wiring has a characteristic impedance three times or more higher than the impedance in the fourth capacitor in a frequency range in which the undesired electromagnetic wave radiation occurs, and the fourth power source wiring has a length which is equal to or larger than a value obtained by multiplying 20 mm by the wavelength reduction rate of the printed circuit board and is equal to or smaller than a value obtained by multiplying one quarter the wavelength at the upper limit frequency at which the undesired electromagnetic wave radiation occurs by the wavelength reduction rate.
As described above, individual decoupling circuits are separately applied to the power source terminal which produces large power source noise and to the other power source terminals, thereby making it possible to provide a printed circuit board which can effectively suppress power source noise, so that a printed circuit board which can suppress electromagnetic wave radiation can be provided.
According to a seventh aspect, the printed circuit board is configured such that an inductor component is used instead of each of the first, second, third and fourth power source wirings.
The use of the inductor component instead of the power source wiring enables a printed circuit board to be provided, wherein only a small mounting space is required and radiation can be suppressed, without increasing cost, even when the length of the power source wiring is limited in wiring design.
According to an eighth aspect, the printed circuit board is configured such that when the spacing between the capacitors or the spacing between the first capacitor and the second, third or fourth capacitor is smaller than a value obtained by multiplying 20 mm by the wavelength reduction rate of the printed circuit board, an inductor components is used instead of the wiring for connecting the capacitors.
Thus, even when the spacing between the capacitors is smaller than the value obtained by multiplying 20 mm by the wavelength reduction rate for ensuring smaller mounting space, by using the inductor component instead of the power source wiring, a printed circuit board can be provided wherein the mounting space can be decreased and radiation can be effectively suppressed at low cost.